1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus to light a discharge lamp for illuminating a liquid crystal display (LCD) device, and more particularly to a discharge lamp lighting apparatus having a dimmer circuit which includes a control means to dynamically control screen brightness according to an input signal, and also a control means to control the screen brightness based on the user's operation, and which enables a dimming operation to be performed over a wide range without generating a brightness gradient even when a long lamp is used.
2. Description of the Related Art
A lighting apparatus such as a backlight device is employed in an LCD device used as a display device for an LCD monitor, an LCD television, and the like. A discharge lamp such as a cold cathode lamp is extensively used as a light source for such a lighting apparatus. A discharge lamp lighting apparatus is usually provided with an inverter circuit which includes a step-up transformer in order to generate a high AC voltage required to light a discharge lamp.
Many LCDs are so structured as to have screen brightness controlled according to the ambient environmental changes, and the like, such that the screen brightness basically is adjusted not only based on the user's operation but also dynamically adjusted according to continually changing input signals, wherein a voltage (or current) value out of a plurality of voltage (or current) values set for achieving respective predetermined screen brightness levels is appropriately selected by a switching operation according to a command signal sent from, for example, a microcomputer. The aforementioned control means to adjust the screen brightness based on the user's operation includes a burst dimming mode in which the oscillation operation of an inverter circuit is forcibly switched on and off so as to vary the ratio between on- and off-periods thereby performing a dimming operation, and the control means to dynamically adjust the screen brightness includes a current dimming mode in which the input DC voltage of an inverter circuit is varied by a DC-DC converter, or the like thereby varying the current of a discharge lamp connected at the secondary side of an inverter transformer.
In the burst dimming mode, the oscillation operation of an inverter circuit is intermittently switched on and off so as to vary the on-duty time (on-period per cycle) of on-off operation thereby controlling the average value of a current flowing in a discharge lamp. And in the current dimming mode, the value of a DC voltage supplied to the inverter circuit is varied so as to control the high current wave value of a lamp current flowing in the discharge lamp thereby controlling the brightness of the discharge lamp.
The current dimming mode has the following problem. When a lamp current is reduced in order to lower the brightness of a discharge lamp, a brightness gradient tends to be caused between the high tension side and the low tension side of the discharge lamp as shown in FIG. 4. Also, if a lamp current is reduced below the guarantee value of the discharge lamp, the discharge lamp exhibits an unstable discharge operation, which causes a problem of flickering, and also causes difficulty of maintaining a stable discharge operation therefore allowing the discharge lamp to readily go out. Consequently, the current dimming mode generally enables a dimming control range of about 100% to 70%.
Under the circumstances, a discharge lamp lighting apparatus is disclosed which employs a current dimming mode as a control means to dynamically adjust the screen brightness according to continually changing input signals, in combination with a burst dimming mode as a control means to adjust the screen brightness based on the user's operation (refer to, for example, Japanese Patent Application Laid-Open No. 2001-357995).
FIG. 6 is a circuit diagram of an exemplar of such a discharge lamp lighting apparatus as described above, and FIG. 7 is a waveform diagram to explain the operation of the discharge lamp lighting apparatus of FIG. 6.
The discharge lamp lighting apparatus of FIG. 6 for lighting a discharge lamp (CFL) 1 includes a DC-AC inverter 2 to supply a driving current to the discharge lamp (CFL) 1, an input section 3 provided with a brightness adjusting means, and a main control section 4 which has a port terminal to output a pulse width modulation (PWM) signal to the inverter 2 in response to the output sent from the input section 3, and also a digital-analog (D-A) terminal to output a reference current value to the DC-AC inverter 2 in response to the output sent from the input section 3.
The discharge lamp lighting apparatus of FIG. 6 performs a dimming operation as follows. Referring to FIG. 7, if a user operates to cause the input section 3 to function to gradually decrease the brightness of the CFL 1 from its maximum level, the main control section 4 functions to gradually decrease the reference current outputted from the D-A terminal until it comes down to a predetermined value. During this process, the output from the port terminal is represented as a PWM signal with 100% on duty (refer to period TK1 in FIG. 7). Then, if the user's operation demands further decrease of the brightness of the CFL 1, the main control section 4 functions to stepwise decrease the on-duty time of the PWM signal outputted from the port terminal while the reference current outputted from the D-A terminal is maintained at the predetermined value (refer to period TK2 in FIG. 7).
Thus, in the discharge lamp lighting apparatus of FIG. 6, the current dimming mode is performed until the reference current, which is outputted from the D-A terminal, comes down to arrive at the predetermined value, and if the reference current having arrived at the predetermined value is caused to further decrease, then the burst dimming mode is performed with the reference current maintained at the predetermined value, whereby a wide range of dimming operation is enabled without causing a brightness gradient as shown in FIG. 5.
Under the circumstances, a large LCD, which has been recently developed for use in, for example, a large television, requires an increasingly longer discharge lamp (e.g., a cold cathode tube), and such an elongated long discharge lamp tends to cause a brightness gradient between the high tension side and the low tension side of the lamp even if the lamp current value is within the guarantee value of the discharge lamp. Consequently, if the current dimming mode is performed in the discharge lamp lighting apparatus of FIG. 6 such that the lamp current is decreased down to the reference current value, there is still a likelihood that the brightness gradient as shown in FIG. 4 will be caused at the CFL 1.